Surface repair tools investigate surface ties to identify minor mismatches between surface boundaries. If these mismatches are not identified and corrected, they will produce defects in the mesh.
A surface tie defines an association between a curve on the boundary of one surface and a curve on the boundary of another surface. When the part is later meshed, the mesher will use the surface tie definitions to correct for the boundary mismatches and ensure that no free edges are created in these areas.